1. Field of the Invention
The present invention relates to a rotational speed detector having a rotor which rotates in accordance with the rotation of a rotating object detector electromagnetically detecting rotational speeds of the rotating object based on the rotation of the rotor.
2. Description of Related Art
A known rotational speed detector the has a rotor which rotates in accordance with the rotation of a rotating object. The rotor includes a rotor switch having a plurality of pairs of portions having different magnetic resistances alternately arranged A, a magnetic circuit device is arranged separately from the rotor and forms a closed magnetic circuit together with the rotor switch A transducer for converts magnetic flux alternation in the closed magnetic circuit to electric signals. For example, the specification of Japanese Utility Model Laid-Open Publication No. Sho-48-43678 discloses a rotational speed detector for a vehicle wheel having a metal rotor with claws which rotate together with a vehicle wheel, A magnetic core and a permanent magnet fixed to a vehicle body forming a closed magnetic circuit together with the claws and an electromagnetic coil for converting alternation of the magnetic flux in the closed magnetic circuit to alternating current.
In this rotational speed detector, two conditions in the closed magnetic circuit appear alternatively according to rotation of the rotor rotated with the wheel. Namely, the closed magnetic circuit is formed to include the claws of the rotor in the first condition while it is formed to include slits located between the claws in the second condition. Under the alternation of the two conditions, the magnetic resistance in the closed magnetic circuit changes, i.e., a magnetic switch formed in the closed magnetic circuit is turned on or off, thereby creating alternation of the magnetic flux in the closed magnetic circuit. According to the magnetic flux alternation, alternating current in the electromagnetic coil is generated. The frequency and voltage of the alternating current vary according to a magnitude of the magnetic flux change which has a certain relation with the rotational speed of the vehicle wheel. Therefore, the rotational speed of the vehicle wheel can be detected according to the alternating current.
For this type of the rotational speed detector, it is desirable to increase its output in order to improve accuracy of speed detection and to detect a lower rotational speed. In the conventional detector described above, one of the magnetic cores forming the closed magnetic circuit is disposed to face teeth or pillars of the rotor and the other core is disposed to face a portion of the rotor where no teeth are formed. That is, there is only one magnetic switch in the closed magnetic circuit. To increase the output of this detector, an necessary to use a larger size of a permanent magnet and an electromagnetic coil.
Therefore, it is the object of the present invention is to provide an improved rotational speed detector having a higher output without increasing the size of the detector.